Ice-Free Arctic in Pliocene, Last Time CO2 Levels above 400 PPM

Scientists trying to determine how the Earth might change as temperatures rise often look back in time to a period around 3.6 million years ago called the middle Pliocene, when concentrations of carbon dioxide ranged from about 380 to 450 parts per million. (Today they are nearing 400.)

A study published yesterday in the journal Science analyzed the longest land-based sediment core ever taken in the Arctic and found that during this period, from 3.6 million to 2.2 million years ago, the area around the North Pole was much warmer and wetter than it is now.

In the middle Pliocene, summer temperatures in the Arctic were around 60 degrees Fahrenheit, which is about 14 degrees warmer than they are now, the study found.

The sediment core came from the bottom of a deep lake in Russia about 60 miles north of the Arctic Circle, called Lake El'gygytgyn.

This lake is special because glaciers did not cover it and shield it from sediment, a process that often removes a lot of the material geologists look for in a sediment core, said Julie Brigham-Grette, the study's lead author and a quaternary geologist at the University of Massachusetts, Amherst.

"I don't want to boast, but I think it is really the only place you could find this in the Arctic," she said. "We've got a record all in one place of the entire story."

It wasn't easy. The effort to get a drill rig over the lake and drill out a sediment core took years, Brigham-Grette said.

The team shipped the rig out from the United States and with the help of Russian scientists finally got it to its remote location in 2008. The drilling took place in the late winter and spring of 2009.

Going back to find clues to the future
After getting the sediment core out of the lake, the scientists analyzed it in two parts. They published the results of their first analysis, looking at the last 2.8 million years, last summer.

The second part of the story involves the part of the core from 2.2 million to 3.6 million years ago. In their analysis, the scientists found this part of the sediment core contains enough fossil pollen and other signs of vegetation to bolster the idea that the mid-Pliocene Arctic was warm and forested, making it highly unlikely that the region had year-round sea ice at that time, Brigham-Grette said.

"If you have a forested Arctic with five different kinds of pine trees ... you can't really have perennial sea ice at the same time," she said.

Other sediment cores from around the Arctic have also found signs of consistent vegetation during the Pliocene, but because they cover shorter time spans, researchers have not been able to pull together a complete picture of what the Arctic was like in terms of vegetation and temperatures over the entire time span.

Some interpretations of ocean cores from the middle Pliocene have suggested that the Arctic did have constant ice during this period, a question Brigham-Grette said now needs to be resolved.

"It's like any other scientific study where you answer some questions that you have and then all of a sudden you have new questions," she said.

Is the Arctic playing catch-up?
Martin Melles, a researcher at the University of Cologne and another author on the study, said he was surprised that the core showed the Arctic climate in the middle Pliocene up to the early Pleistocene was so warm and wet.

"This suggests to me that the Arctic today is not in equilibrium with the greenhouse gas settings, meaning that we have to expect significant increases in [Arctic] temperature and precipitation even if the level of greenhouse gases keeps constant," Melles said.

The Arctic may eventually catch up to the changes today's rapid CO2 emissions are setting in motion.

"The Earth's system is kind of a sluggish beast. It takes a while for the ocean and atmosphere to respond to this rapid rise in carbon dioxide," Brigham-Grette said.

Brigham-Grette said this paper provided an overview of this part of the sediment core, a sort of "companion piece" to the results from the earlier part of the record published last year.

The team will now start teasing out more details from the core, using new techniques such as one that looks at fossil leaf wax in the core to learn more about precipitation in the past.

Another area of interest is understanding what was happening in Antarctica at the same time in history. This might help scientists understand how different parts of the globe react to climate change.

"One of the things that we are interested in is trying to determine how widespread climate change affects regional stories," Brigham-Grette said.